In the known prior art, during the cutting operation using a motor controllable by a numerical control system as the drive source, a load current flowing through the motor is displayed in the form of a bar graph, or alternatively the fluctuation waveform of the motor load current with a lapse of time is displayed in a graph so that the operator can grasp the cutting state. When the motor load current is displayed in a bar graph, the operator reads a point being machined from the abscissa of the graph while reading a motor load current value shown in the ordinate of the graph, to verify the motor load current values at various machined points, thereby grasping the cutting state. Also, in the case where the fluctuation waveform of the motor load current is displayed in a graph, the cutting state can be grasped based on the elapsed time (corresponding to the machined point) and the motor load current value, respectively shown along the axis of abscissas and axis of ordinates of the graph.
According to the conventional loaded condition plotting method, it is, however, difficult in that the operator must simultaneously and promptly grasp the motor load current value and the point being machined with accuracy. Due to this, even though a disorder is found in the motor load current value, the operator cannot promptly judge the machined point where such disorder has occurred.